Multistage polymerisation processes are well known and widely used in the art for polymerising polypropylene. Process configurations containing at least one slurry phase polymerisation reactor and at least one gas phase polymerisation reactor are disclosed e.g. in U.S. Pat. No. 4,740,550, and further e.g. in WO98/058975 and WO98/058976. A prepolymerisation reactor is often included in the process configuration, typically to maximise catalyst performance. The use of prepolymerisation also avoids overheating the catalyst particles. Prepolymerisation also helps to ensure a more even polymerisation on the catalyst particles reducing the probability of creating fines in later reaction steps.
Single site catalysts have been used to manufacture polyolefins for many years. Countless academic and patent publications describe the use of these catalysts in olefin polymerisation. One big group of single site catalysts are metallocenes, which are nowadays used industrially and polyethylenes and polypropylenes in particular are often produced using cyclopentadienyl based catalyst systems with different substitution patterns.
Single site catalysts are used in propylene polymerisation in order to achieve some desired polymer properties. However, there are problems in using single site catalysts on industrial scale in multistage polymerisation configurations.
Metallocenes are conventionally supported on a carrier such as silica. Research has found that heterogeneous catalysis (in which the catalyst particles do not dissolve in the reaction medium) gives rise to more advantageous polymer products than homogeneous catalysis (in solution). The use therefore of a support is common place. The use however of supported catalysts is associated with problems such as silica residues in the final product. Further, there is still room for improved activity, and improved polymer particle formation.
In WO03/051934, the inventors proposed an alternative form of catalyst which is provided in solid form but does not require a conventional external carrier material such as silica. The invention is based on the finding that a homogeneous catalyst system containing an organometallic compound of a transition metal can be converted, in a controlled way, to solid, uniform catalyst particles by first forming a liquid/liquid emulsion system, which comprises as the dispersed phase, said solution of the homogeneous catalyst system, and as the continuous phase a solvent immiscible therewith, and then solidifying said dispersed droplets to form solid particles comprising the said catalyst.
The invention described in WO03/051934 enabled the formation of solid spherical catalyst particles of said organotransition metal catalyst without using e.g. external porous carrier particles, such as silica, normally required in the art. Thus, problems relating to catalyst silica residues can be solved by this type of catalyst. Further, it could be seen that catalyst particles having improved morphology, will give, due to the replica effect, polymer particles having improved morphology as well.
Some multistage polymerisations utilise a slurry then gas phase set up. One of the possible limitations of polymerization catalysts in general, and of metallocene-based catalysts in particular, is that when the catalyst has a high activity in slurry, e.g. bulk, the activity in gas phase is often low. This makes it difficult to achieve a low bulk-to-gas phase ratio of the produced material (the so-called bulk/GP split). In other words, strong (initial) activity in the bulk step can lead to faster catalyst deactivation, in turn leading to a poorly active catalyst in the gas phase reactor. This problem is especially relevant for gas phase copolymerizations in a three-step sequence, slurry phase, gas phase, gas phase where the copolymer is produced in the third step.
To be relevant for industrial polypropylene production, a single site catalyst must have good performance under all polymerisation conditions, in particular in conditions, where polymerisation temperature is at least 70° C., and in all actual polymerisation reactors of the multistage process including both liquid (bulk slurry) and gas phase reactors. The present invention tries to address this issue.
The present inventors have now found that when all or most of the hydrogen needed to control the MFR of the polymer produced in the first main polymerization reactor is fed directly into the first main polymerization reactor (usually a slurry bulk step) of the polymerisation process, instead of into the prepolymerisation step, the productivity (activity) of the catalyst in the subsequent gas phase step, and in particular a second gas phase step, is much higher than in the case where a significant portion or all hydrogen is fed to the prepolymerisation step. Overall productivity remains approximately on the same level but the activity within different reactors changes. This allows therefore an increase in the amount of polymer made in the gas phase relative to the amount of polymer made in the bulk phase—we address the bulk/GP split.
The inventors have found that the way hydrogen is distributed between the prepolymerisation and the bulk steps has clear influence on the productivities of each individual step, in particular the subsequent one or more gas phase steps. Through manipulation of hydrogen therefore, the person skilled in the art can affect the composition of the final polymer.
Thus, by changing how hydrogen is fed, that is by varying the amount of hydrogen fed in a prepolymerisation step and the amount fed into the bulk step, the catalyst productivity can be varied in each polymerisation step (although not independently). This means that, for a polymer produced in a 3-step process (i.e. a process comprising three actual polymerisation steps), such as a heterophasic propylene ethylene copolymer composition, the productivity in the second gas phase step, in which the C2/C3 copolymerisation takes place, can be increased.
The catalysts of most interest in the process of the invention are described in WO2013/007664 and WO2013/007650. Both these documents also consider the formation of heterophasic propylene ethylene copolymers based on a slurry (bulk) and single gas phase reaction. In WO2013/007664 and WO2013/007650 however, hydrogen is used in the prepolymerisation step and very small amounts of hydrogen are added at the start of the bulk polymerisation with no further hydrogen added. There is no appreciation therefore of the importance of hydrogen levels between prepolymerisation and the first main polymerisation step in terms of achieving a different balance of activity in the reactors and hence a broader window of operation and hence a manipulable composition in the final polymer.
The present inventors therefore enable a process in which the ratio of the material produced in each step can be changed, and especially the amount of copolymer produced in the last reactor of a three-reactor slurry bulk/gas phase/gas phase process can be increased without the need for increasing the residence time in this last reactor.